The superheterodyne receiver is a receiving system that performs a frequency conversion on a received signal as part of the detection process. That is, the received signal is converted from a first frequency range (i.e., the RF frequency) to a different, second frequency range (i.e., the intermediate or IF frequency) in a frequency conversion device, such as a mixer. The IF frequency can be greater than the RF frequency, in which case the receiver is known as an up-converter system, or the IF frequency can be lower than the RF frequency, in which case the receiver is known as a down-converter system.
The direct conversion receiver (DCR) is a special case of the superheterodyne receiver where the IF signal is located at substantially zero frequency (i.e., direct current or DC). To implement a direct conversion receiver, a local oscillator frequency that is substantially equal to the desired RF frequency is applied to the first mixer stage. Direct conversion receivers are generally attractive for many reasons, including extremely wide frequency range, simplicity, and monolithic integration of a majority of the circuits (such as, for example, the IF gain and filter selectivity circuits). However, direct conversion receivers suffer from a problem that is avoided in conventional superheterodyne receivers that do not convert to DC. That is, strong amplitude varying signals (such as pulsed or amplitude modulated carriers) in the vicinity of the receiver and at frequencies within or near the RF preselector pass-band can be detected in the nonlinear elements of the mixer (by the square law process). These interfering signals can show up at the output of the mixer as low frequency signals around DC. In a conventional superheterodyne receiver, these interfering signals (which will be referred to herein as amplitude modulated interference or AMI) are rejected in the IF band-pass filter, which is tuned to pass only a range of frequencies centered on the IF frequency (which is much higher than the frequency of the detected AMI baseband signal). In a DCR, however, the IF filter is a low-pass filter that often passes the AMI to an output. The presence of this AMI in the IF output signal can seriously degrade the performance of the DCR.
Therefore, there is a need for a method and apparatus for reducing the impact of AMI in a direct conversion receiver.